1. Field of the Invention
The present invention relates to a glass-ceramic composition (a composition for preparing a substrate body) for ceramic electronic parts, a ceramic electronic part, and a method for manufacturing a multilayer ceramic electronic part. In particular, the present invention relates to a glass-ceramic composition that can be sintered at a low temperature of not more than about 1,000xc2x0 C., a ceramic electronic part such as a multilayer ceramic substrate and a thick-film hybrid circuit part that is composed of the composition, and a method for manufacturing a multilayer ceramic electronic part.
2. Description of the Related Art
Multilayer ceramic electronic parts such as multilayer ceramic substrates are used in various electronic apparatuses, including mobile information terminal devices and personal computers, for example. The multilayer ceramic electronic parts have an advantage in that they make it possible to miniaturize the electronic apparatuses and to realize a higher wiring density.
The multilayer ceramic electronic parts have metal wiring conductors on the inside, and therefore, the metal wiring conductors are exposed to the same baking conditions in the baking step as the ceramic substrate bodies to be used for the multilayer ceramic electronic parts. Accordingly, metal materials constituting the metal wiring conductors must stand such simultaneous baking.
Conventionally, alumina is generally used as a material for a substrate body for use in the multilayer ceramic electronic parts. Since a temperature as high as about 1,600xc2x0 C. is necessary for baking alumina, a high melting-point metal such as tungsten and molybdenum must be used as the metal material for the metal wiring conductors that are to be baked at the same time. However, since a metal as tungsten or molybdenum has a high electric resistance and alumina has a high dielectric constant, the requirement of higher frequencies cannot be fully met at present by electronic apparatuses in which such multilayer ceramic electronic parts are used.
Accordingly, it is desired to use such a metal as gold, silver, copper, silver-palladium, silver-platinum, nickel or aluminum that has a relatively low electric resistance for the metal wiring conductors. However, since these metals have a relatively low melting point, it is necessary that materials for the substrate bodies must be capable of being baked at a temperature of not more than about 1,000xc2x0 C. Glass-ceramic materials are supplied for practical use, as a representative of the materials for preparing substrate bodies that can be baked at such a relatively low temperature.
In general, when multilayer ceramic electronic parts such as multilayer ceramic substrates are manufactured using a glass-ceramic material as a material for preparing substrate bodies, the glass-ceramic slurry is prepared by fully mixing a glass powder and a ceramic powder together with an organic vehicle at a specific ratio, and is subjected to a sheet forming process according to a doctor blade method or the like so as to prepare glass-ceramic green sheets. Next, electroconductive films and electroconductive via holes that are to act as metal wiring conductors are formed on specific glass-ceramic green sheets by means of screen printing or the like. Then, these plural glass-ceramic green sheets are laminated (stacked) to form a green laminate and the green laminate is subjected to pressing, followed by baking, to form a multilayer ceramic electronic part such as a multilayer ceramic substrate. Regarding the electroconductive films constituting part of the metal wiring conductors, those located on the external surfaces may be formed after the baking.
As a material for the above-described metal wiring conductors, for example, gold, silver, copper, silver-palladium, silver-platinum, nickel, aluminum or the like is used as described above. Especially, silver or silver alloys are advantageously used as a material for the metal wiring conductors since they have a small specific resistance and they can be baked in air.
Regarding a glass-ceramic composition that can be sintered at a low temperature of not more than about 1,000xc2x0 C. and that can be baked at the same time with metal wiring conductors containing a low-resistance metal such as silver or copper, a glass-ceramic composition having the following constitution is proposed in Japanese Published Patent Application No. 2000-351688 (corresponding to U.S. application Ser. No. 09/550,826 which is now U.S. Pat. No. 6,376,055, and EP 1059271): a borosilicate glass powder comprising from 5% to 17.5% by weight of B2O3, from 28% to 44% by weight of SiO2, from 0% to 20% by weight of Al2O3 and from 36% to 50% by weight of at least one of CaO, MgO and BaO, as well as a ceramic powder such as an alumina powder.
When a substrate body is prepared by using such a glass-ceramic composition, it can have a high mechanical strength and a low dielectric constant, and therefore, a ceramic electronic part such as a multilayer ceramic substrate having good properties and high reliability can be obtained by using this substrate body. Furthermore, since the substrate body can have a coefficient of thermal expansion of not less than 6.0 ppm/xc2x0 C., matching the coefficient of thermal expansion with that of a printed wiring substrate made of an epoxy resin or the like is well-established, and accordingly, a high connection reliability can be achieved.
However, when a glass-ceramic composition having such a specific constitution is used for manufacturing ceramic electronic parts, the substrate body made from the composition sometimes develops a gray color instead of a white color that is an attribute that the substrate body is expected to have inherently. It is considered that this is caused by the fact that carbon generated from an organic binder contained in the glass-ceramic green sheets is not completely burnt out in the baking step, and is left in the substrate body.
Furthermore, there are occasions when discoloration into yellow or other colors occurs in the vicinities of the metal wiring conductors of ceramic electronic parts. This phenomenon occurs particularly when the metal wiring conductors are formed from a silver-containing metal, and it is supposed to be caused by diffusion of silver from the metal wiring conductors into the substrate body during the baking step.
The above-described gray discoloration of the substrate body and discoloration into yellow or other colors in the vicinities of metal wiring conductors do not exert direct influence on the electrical properties of the obtained ceramic electronic parts. However, in some cases, such discoloration will result in fluctuation in color of the merchandise, degrading its commercial value.
Accordingly, it is an object of the present invention to provide a glass-ceramic composition for ceramic electronic parts, a ceramic electronic part formed by using this composition, and a method for manufacturing a multilayer ceramic electronic part, in order to solve the problems described above.
According to one aspect of the present invention, provided is a glass-ceramic composition for ceramic electronic parts comprising a borosilicate glass powder comprising from about 5% to 17.5% by weight of B2O3, from about 28% to 44% by weight of SiO2, from 0% to about 20% by weight of Al2O3 and from about 36% to 50% by weight of MO where MO is at least one selected from the group consisting of CaO, MgO and BaO, as well as a ceramic powder, and for solving the above-described technical problems, the present invention is characterized in that the composition also contains an additive powder containing at least one selected from the group consisting of cerium oxide, bismuth, bismuth oxide, antimony, and antimony oxide.
The content of the above-described additive powder is preferably in the range of about 0.005% to 5% by weight based on the total weight of the borosilicate glass powder and the ceramic powder. It is more preferably in the range of about 0.01% to 1% by weight.
The average particle size of the additive powder is preferably in the range of about 0.1 to 20 xcexcm.
The ratio of the borosilicate glass powder to the ceramic powder is preferably from about 40:60 to 49:51 by weight.
The ceramic powder preferably contains an alumina powder.
According to another aspect of the present invention, a ceramic electronic part is provided that comprises a substrate body obtained by molding from the above-described glass-ceramic composition, followed by baking, as well as metal wiring conductors formed in association with (for example, on, in, through, between, and/or over) the substrate body and baked at the same time with the composition.
The ceramic electronic part may be a multilayer ceramic electronic part wherein the substrate body has a lamination structure.
Furthermore, the coefficient of thermal expansion of the substrate body for ceramic electronic parts is preferably not less than about 6.0 ppm/xc2x0 C.
The present invention is especially advantageous when the metal wiring conductors contain silver.
According to still another aspect of the present invention, a method for manufacturing a multilayer ceramic electronic part is also provided. The method for manufacturing a multilayer ceramic electronic part according to the present invention is characterized in that it comprises the step of preparing a glass-ceramic slurry by adding an organic vehicle to the above-described glass-ceramic composition, the step of preparing an electroconductive paste comprising a metal powder, the step of preparing a green laminate comprising a plurality of glass-ceramic green layers formed and laminated using the glass-ceramic slurry, and metal wiring conductors formed by applying the electroconductive paste to specific members of the glass-ceramic green layers, and the step of baking the green laminate.
To this method, a so-called shrinkage-free process may be applied wherein shrinkage in the direction of the main surface of the glass-ceramic green layers is practically prevented from occurring in the baking step. In this case, the manufacturing method has an additional step of preparing an inorganic material slurry comprising an inorganic material powder that is not sintered at the temperature for sintering the glass-ceramic composition, the step of preparing the green laminate includes the step of forming a constraint layer(s) comprising the inorganic material slurry so that the layer(s) are contacted with the main surface(s) of a specific member(s) of the glass-ceramic green layers, and the step of baking the green laminate is performed at a temperature that causes sintering of the composition but does not cause sintering of the inorganic material powder.
When the above-described shrinkage-free process is applied to the method for manufacturing the multilayer ceramic electronic part, it is preferable to form the constraint layers so that they are located on both ends in the lamination direction of the glass-ceramic green laminated layers during the step of preparing the green laminate, and then to remove the constraint layers after the baking step of the green laminate.